1. Field of the Invention
The present invention relates generally to electrical circuits, and more particularly but not exclusively to low dropout regulators.
2. Description of the Background Art
A low dropout (LDO) regulator is a linear DC voltage regulator with relatively small input-output differential voltage. A low dropout regulator typically includes an error amplifier driving an output transistor. The error amplifier compares the output voltage of the regulator with a reference voltage to generate a signal that controls the output transistor to maintain the output voltage within regulation requirement. The regulator's dropout voltage is the minimum voltage across the regulator required to maintain the output voltage at the correct level.
A compensation network may be used to stabilize the response of a low dropout regulator. Without a compensation network, the regulator may consume large quiescent current that may place internal poles of the transfer function (“pole”) of the regulator to high frequency. Furthermore, without a compensation network, the regulator will only be stable in a narrow load current range.
A fixed RC (resistor-capacitor) compensation network is better than using no compensation network at all. However, with a fixed RC compensation network, a low dropout regulator may be unstable at load current extremes, such as with very low or very large load current. A typical fixed RC compensation network is only stable in a narrow current load range.
C. Shi, B. Walker, E. Zeisel, B. Hu, G. McAllister, “A Highly Integrated Power Management IC for Advanced Mobile Applications,” IEEE 2006 Custom Intergrated Circuits Conference (CICC) discloses dynamic resistor compensation for a low dropout regulator. While potentially promising, dynamic resistor compensation is relatively difficult to realize in actual circuits, makes it relatively difficult to track load current, and introduces a very low pole in the error amplifier when load current is low, thereby adversely affecting power supply ripple rejection (PSRR).